The present invention relates generally to image forming apparatus, and more particularly to an image forming apparatus which is capable of forming an image on a photosensitive medium at a number of different recording dot densities.
First, a description will be given of an optical scanning part of a conventional laser beam printer or digital copying machine. FIG. 1 generally shows the construction of this optical scanning part. A light beam is emitted from a laser diode (LD) 1 which is used as a light beam source, and this light beam is collimated by a collimate lens 2 into a collimated beam. An extra part of the collimated beam is eliminated from the light beam from the collimate lens 2 by an aperture 3, and the light beam is shaped into a shaped light beam. The aperture 3 has a slit with a size predetermined depending on the diameter of a dot of an image which is formed on a photosensitive medium 5. The shaped light beam passing through the aperture 3 is focused on a deflection surface of a polygonal mirror 6 by a cylindrical lens (#1) 4. The light beam from the cylindrical lens 4 is deflected on the deflection surface of the polygonal mirror 6, which is used as a deflector, toward the photosensitive medium 5, and is scanned in a main scan direction as indicated by an arrow x in FIG. 1, (in a longitudinal direction of the photosensitive medium 5) by rotation of the polygonal mirror 6. An f0 lens 7 allows the light beam from the polygonal mirror 6 to scan the photosensitive medium 5 at a constant speed in the main scan direction x, and serves to compensate for scanning distortion. The light beam from the f0 lens 7 is reflected on a mirror (#2) 8 to the photosensitive medium 5, and this reflected beam is focused by a cylindrical lens (#2) 9 in a sub scan direction, as indicated by an arrow y in FIG. 1. A sub scan is made in the sub scan direction y by rotation of the photosensitive medium 5 in step with the rotation of the polygonal mirror 6 for making a main scan of the light beam in the main scan direction x on the photosensitive medium 5. The thus converged light beam is irradiated on the photosensitive medium 5 to form an electrostatic image thereon.
On the other hand, a light detector 13 detects a light beam from the laser diode 1 once per scanning line, and supplies a synchronization control signal to a synchronization control part of the laser beam printer so as to control the timing with which the turning ON of the laser diode 1 starts. This light beam which is deflected on the surface of the polygonal mirror 6 is reflected by a mirror (#3) 10, and is focused by a cylindrical lens (#3) 11. The focused light beam passing through the cylindrical lens 11 is directed to the light detector 13 through an optical fiber 12.
However, the optical scanning part of the laser beam printer described above is constructed so as to for an electrostatic image on the photosensitive medium with dots having a fixed recording dot density D. For making a recording dot density D of the formed image variable, it is required to change a hardware of the optical scanning part including the aperture 3 an the cylindrical lens 4. And, to ensure good printing quality, it is important to change from the dot diameter of the image before the recording, dot density D is varied.
FIGS. 2A to 2D show a few examples of dot patterns when printing is made with different recording dot densities of 200 d (dots per inch) and 400 dpi. As shown in FIG. 2A and 2B, to ensure good printing quality when the recording dot density of 400 dpi as shown in FIG. 2A is changed to 200 dpi as shown in FIG. 2B, it is necessary to change also the 400 dpi dot diameter to the 200 dpi dot diameter. The adjacent dots of the image overlap each other appropriately. In FIG. 2C, however, the adjacent dots overlap excessively because printing is made at 400 dpi pitch with a 200 dpi dot diameter used. And, in FIG. 2D, printing is performed at 200 dpi pitch with a 400 dpi dot diameter used, and white blank spaces are left between the adjacent dots of the formed image. Thus, the printing quality of FIGS. 2C and 2D is worse than that of FIGS. 2A and 2B. Therefore, it should be noted that, to attain good printing quality when the recording dot density is changed, the dot diameter must also be changed in accordance with the changed recording dot density.
For making the dot pitch variable, there are several points which must be considered.
In performing the printing with the optical scanning part described above, a scanning frequency fv (Hz) for a scanning line is represented by the following formula. EQU fv=(D/25.4) V
In this formula, D is a recording dot density (number of dots per inch) and V is a linear speed of a photosensitive medium (mm/s). A rotating speed Rm (rpm) of a polygonal mirror is represented by the following formula. EQU Rm=fv(60/n)=(D/25.4)(V/n) (1)
In this formula, n is the number of surfaces of the polygonal mirror. Therefore, EQU D oc Rm/V (1)'
A clock signal frequency W (Hz) is represented by EQU W=(fv/Er)(D/25.4)L
In this formula, Er is an effective scanning rate (0&lt;Er&lt;1) at which a main scan is made by a polygonal mirror, and L is an effective scanning width, and substituting (1) into the above formula makes EQU W=(D/25.4).sup.2 (1/Er)(V/n)L (2)
Therefore, ##EQU1##
To make the dot pitch or recording dot density variable, it is necessary to vary at least the photosensitive medium's linear speed V, the polygonal mirror's rotating speed Rm, and the clock signal frequency W.
In addition, in the image forming apparatus according to the present invention which uses an electrophotographic process for forming an electrostatic image, a light beam from the laser diode 1 is irradiated on a surface of the photosensitive medium 5 which surface of the photosensitive medium 5 is charged at a certain voltage level. The voltage of the charged photosensitive medium 5 is determined depending on the quantity of the irradiated light (or, light energy) supplied to the photosensitive medium 5. FIG. 3 generally shows the light attenuation characteristic of the light which is irradiated on the photosensitive medium 5. The relationship between the light energy P (erg/cm.sup.2) and the recording medium voltage Vr is as shown in FIG. 3, and it is desirable to apply a voltage selected from in a saturation range, as indicated by an arrow A in FIG. 3, to the photosensitive medium 5. In this saturation range, the voltage applied to the photosensitive medium 5 must be in a saturated state. Once the voltage applied to the photosensitive medium 5 is given, the light energy J supplied to the laser diode 1 is determined in the basis of the light energy vs. recording medium voltage characteristic curve as shown in FIG. 3.
This light energy J which is given to the photosensitive medium 5 is proportional to a luminous power "P" of a light beam from the laser diode 1 and to a turning ON time "t" for which the laser diode 1 is turned ON. This is represented by: EQU J oc P t (3)
In this formal, it should be noted that, when the recording dot density D is varied and the clock signal frequency W is varied, the turning ON time t is accordingly changed because the turning ON time t is changed in proportion to the clock signal frequency W. Although the dot pitch may be changed to have an unchanged light energy per unit area, the luminous power P is varied when the recording dot density D is varied. Thus, in order to keep the light energy J constant, it is necessary to vary appropriately the quantity of light or luminous power of a light beam emitted from the laser diode 1. And, to change the dot diameter, it is necessary to change the size of the opening of the aperture 3 through which the light beam from the laser diode 1 passes. When the size of the opening of the aperture 3 is great, the light energy applied to the photosensitive medium 5 becomes great. Hence, in order to keep the light energy to the photosensitive medium 5 constant, the quantity of light supplied from the laser diode must be reduced.
As described above, for making the recording dot density variable, it is necessary to control suitably the clock signal frequency W, the polygonal mirror rotating speed Rm, the photosensitive medium linear speed v and the laser diode's luminous power P. However, in the conventional image forming apparatus, there is a problem in that, for making the recording dot density D variable, the hardware of the image forming apparatus must have a complicated structure, the manufacturing cost must be increased, and a plurality of variable elements must be prepared for a single optical scanning system which uses several different recording dot densities.